1. Field of the Invention
The present invention relates to methods of processing sound signals in hearing aids. The invention more specifically relates to a method of processing sound in a hearing aid with a compressor that is active at very low sound levels. The invention, still more specifically, relates to a method of processing sound in a hearing aid that alerts the user of the occurrence of a sudden sound in a stationary sound environment.
2. The Prior Art
As used in this context, a hearing aid is understood as generally comprising a device with an input transducer for transforming an acoustic input signal into a first electrical signal, a signal processor for generating a second electrical signal based on the first electrical signal, an output transducer for conversion of the second signal into sound, and a battery for supplying energy to the signal processor.
Typically, a hearing aid has a housing holding the input and the output transducer, the battery and the signal processor. The housing is adapted to be worn, i.e. behind the ear, in the ear, or in the ear canal, and the output of the output transducer is led to the eardrum in a way that is well-known in the art of hearing aids. The processor will generally be adapted for processing the electric signal in order that the resulting acoustic output signal compensates a hearing deficiency of a user.
U.S. Pat. No. 4,777,474 provides an alarm system for the hearing impaired, comprising a base station radio transmitter adapted to transmit, upon detection of an alarm state, a signal to a portable unit. The portable unit includes all parts of an ordinary hearing aid together with a radio receiver to receive the signal transmitted by the base station.
WO 99/34642 discloses a hearing aid with an automatic gain control, effected by detecting an input sound level and/or an output sound level and adapting the output sound level supplied by the hearing aid in response to the detected sound level by controlling the gain of the hearing aid towards an actual desired value of the output sound level. The gain control is effected at increases and decreases, respectively, of the input sound level by adjusting the gain towards the actual desired value with an attack time and a release time, respectively, which are adjusted in response to the detected sound level to a relatively short duration providing fast gain adjustment at high input and/or output sound levels and to a relatively long duration providing slow gain adjustment at low input and/or output sound levels.
It is well known in the art to provide a hearing aid having a compressor with a characteristic that has two linear segments that are interconnected at a knee-point. The knee-point is typically placed at 50 dB SPL input level, close to the level of normal speech in order to allow a high level of amplification of speech. Below the knee point, the linear segment has substantially no compression, i.e. the gain is a constant gain adapted for compensating the hearing loss at low input signal levels. Above the knee point, the segment has a compression ratio above 1, typically 2:1, for compensating for recruitment. Recruitment is a sensorineural hearing loss whereby loudness increases rapidly with increased sound pressure just above the hearing threshold and increases normally at high sound pressures.
Many hearing aid users being situated in a stable sound environment desire to be able to hear a faint, sudden change in the sound environment, such as a sudden occurrence of a faint sound. For example, being at home, a hearing aid user may desire to be able to hear that a baby starts crying, or that water starts running, that somebody is present at the door, etc. The hearing aid user can increase the gain of the hearing aid to accomplish this but then the hearing aid user may be bothered by other sounds in the stationary sound environment, such as the sound of a ventilator, traffic noise, etc, that might then also be amplified to surpass the hearing threshold. The hearing threshold is the lowest sound level at which sound is perceptible.